Color filters can be very useful for enhancing your views of the Moon and the planets. Depending on the atmospheric conditions, the telescope being used, the observer's experience, and the planet being observed or photographed, the advantages of color filters can be anywhere from subtle to dramatic. The right filter can make all the difference between seeing several small craters in the floor of Clavius on the Moon or not; seeing five or six swirls in Jupiter's belts or not; or seeing the inner Crepe Ring of Saturn or not.
GSO's 1.25" color/planetary filters are manufactured from the purest optical glass and are dyed-in-the-mass (not simply "color coated") for premium performance. They thread into the barrels of virtually any 1.25" telescope eyepiece. Each filter cell has a male and female 1.25" filter thread (M28.5x0.6) on opposite sides, so multiple filters can be stacked to achieve selective filtration of the visual color spectrum. The clear aperture is 25.6mm and each filter comes in a protective plastic storage case. Made in Taiwan.
The #38A Dark Blue Filter is a popular choice for the study of Jupiter's disc because it strongly rejects red and orange wavelengths in the belts, thus enhancing the boundaries between the reddish belts and the adjacent bright zones. It is useful for viewing the Great Red Spot. The filter works well on Martian surface phenomena like dust storms, and is very useful during the violet clearing and for polar caps.
The dark Blue filter also increases the contrast in the rings of Saturn and in low-contrast features between the zones. This is a good one to use on Venus, the low transmission increasing the contrast of dark shadings in the upper clouds. The #38A should only be used on telescopes of 8" of aperture or more, because of the reduced light transmission.
This filter is the best one to bring out the gas tails in comets.
Agena Informational Article: Choosing a Color / Planetary Filter